U.S. Pat. No. 4,124,732, incorporated herein by reference for all purposes, discloses the system utilized to attach the insulating tiles to the space shuttle orbiter. The tiles are isolated from the structure by an intermediate layer of fibrous material forming a strain isolation pad, the fibers of which yield to minimize the stresses which arise from expansion or contraction of the metallic structure and/or mechanical deflections. This strain isolation pad is formed of fibers of high temperature, stable material, which are closely arranged and randomly oriented and then needled to provide a felt-like configuration. The pad is attached to the tile and to the structure by thin layers of a high temperature, silicone adhesive.
It was recognized that the tiles would be subject to various loads during flight. The loads would be particularly high during launch when the tiles will be subject to high vibration, structural bending, and shock loads. Accordingly, tests were run to verify the strength of the system.
During flatwise tensile stress tests, it was found that there was an interfacial failure between the tile and strain isolation pad at low levels. The interfacial failure occurred at 6 psi whereas the predicated failure was at least 13 psi. Failure and consequent loss of tiles could result in a catastrophe since the tiles provide thermal protection to the shuttle during reentry.
Since interfacial failure at 6 psi was below the acceptable minimum, it could not be tolerated, and thought was given to returning to the method of installing tiles disclosed in U.S. Pat. No. 3,920,339. In such method the tile is bonded to a special rigid strain arrestor plate. While such method may be a viable solution if the structure to which the tile is to be attached is flat, it is almost unworkable where there is curvature, such as, in the case of an airframe.
It appeared that the interfacial failure was cased by uneven tension exerted by the strain isolation pad. One reason may be that individual vertically extending fibers were loading up first, resulting in excessive loading in small areas and eventual interfacial failure. Since this failure was occurring at a load less than the internal strength of the tile itself, it was felt that if the load could be transferred more uniformly into the tile material the system would be strengthened.